The present invention generally relates to copying apparatus and more particularly, to an automatic exposure adjusting device for use in a variable copying magnification type copying apparatus enabling change of its copying magnification.
Conventionally, in copying apparatus, it has been so arranged that the exposure amount is roughly adjusted visually by an operator by means of an exposure amount adjustment dial provided on an operating panel. Therefore, copies obtained from a single original document by different operators have different degrees of copying brightness. Furthermore, since further adjustments are required in the case where the copying apparatus are provided with an automatic document feeder for automatically feeding original documents continuously, such a problem frequently arises that a plurality of improper copies are produced before one proper copy is obtained.
Recently, in order to obviate the above described drawback of the known copying apparatus, there have been proposed copying apparatus provided with a density detection means for detecting photographic density of original documents or further with an auutomatic exposure adjusting means for automatically adjusting exposure amount in accordance with the detected photographic density of the original documents obtained by the density detection means. However, in this prior art density detection means, since one or a plurality of photo detectors arranged in parallel with each other are provided in an optical path of an optical system for forming images on a photosensitive member, the optical path is partially intercepted by the photo detector or the photo detectors. Therefore, the prior art density detection means has such disadvantages that unsatisfactory images are formed on the photosensitive member and an excessive load is applied to control members such as a light source, etc. Furthermore, prior art density detection means is so inconvenient that, in order to process a detection value of the photo detector, an arithmetic circuit corresponding to an averaged light measuring method or a selective light measuring method, etc. is required to be provided.
Furthermore, conventionally, in the case of automatic exposure adjustments in variable copying magnification type copying apparatus the relationship between the detection value of the photo detector and the exposure amount of the photosensitive member from an identical original document undergoes change upon change of the copying magnification. Thus, when the copying magnification is changed, it is necessary to mechanically displace the photo detector to a position corresponding to the changed copying magnification or to process the detection value of the photo detector into a value corresponding to the changed copying magnification by using a complicated arithmetic circuit so as to control quantity of light of the light source on the basis of the value. Thus, the known exposure adjusting devices are disadvantageous in that the configuration of the arithmetic circuit becomes sophisticated, thereby making the maintenance troublesome. Moreover, the known exposure adjusting devices have such a drawback that since change of the relation between the detection value of the photo detector and the exposure amount of the photosensitive member upon change of the copying magnification is not uniform, it is extremely difficult to perform the arithmetic operation accurately.
In addition, prior art copying apparatus have such inconveniences that since special measures against abrupt changes of ambient conditions such as temperature, humidity, etc. or deterioration of the photosensitive member, developer, etc. are not taken, it is impossible to obtain proper copies at the time of occurrence of such changes, thus resulting in defective copies having obscure images, images of insufficient gradation at the shadow portion, or photographic fog in the background.